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History of the Dynamo.

549

(12.) Hydraulic Transmission.ŚRefer to Chapters VII.
and XI.

(13.) Electric Transmission can only be briefly described
Faraday, in 1831, discovered magnetic induction, by which a
current is generated in a closed circuit wound on a bobbin, when
the latter is moved before the poles of a permanent magnet
(A, Fig. 565.) Pixii, Clarke, and others thereupon, in 1832,
devised the magneto electric battery B, where the bobbins are
rotated, and introduced the commutator to reverse the alternate
currents formed at c and thus ' straighten out' the total current.
Nollet, Van Malderen, and De Meritens improved this machine
up to the year 1871, dispensing with commutator, and thus pro-
ducing alternating currents (D). Dr. Siemens devised the H
armature E in 1857, working with compound magnet, and in
1866 Wilde employed a small Siemens machine F with commuta-
tor to excite the electro-magnets G of a much larger machine, and
thus avoid the necessity for large permanent magnets- The pro-
gress now was very rapid, and in 1867 Siemens, Wheatstone, and
Varley separately discovered the 'dynamo-electric principle/
by which the machine was made wholly self-exciting, the mere
residual magnetism in the soft iron core, whether new or after
use, being sufficient to commence the current, which then
gradually increased up to its maximum. K is a Siemens dynamo
with H armature and commutator, the currents being thereby